The subject invention is directed to the art of ferrule type tube fittings. More particularly, the invention concerns a two ferrule fitting wherein the rear ferrule is designed to reduce the torque required to rotate the associated nut and to also reduce galling between the rear ferrule and the interior surface of the drive nut. The invention may also be applied in a single ferrule fitting.
A commercially available and highly successful two ferrule fitting used for tubing is illustrated in FIGS. 1 and 1A. FIG. 1 shows the fitting components in a finger tight position preparatory to final tightening, whereas FIG. 1A shows the fitting after final tightening. As shown, the fitting comprises a body 10 having a cylindrical opening 12 counterbored for receiving tube end 13. A tapered, frusto-conical camming mouth 14 is located at the axial outer end of the counterbore. A front ferrule 16 having a smooth, cylindrical inner wall 18 is closely received on the tube. The front ferrule has a frusto-conical outer surface 20 to be received in the camming mouth.
Associated with the front ferrule 16 and located axially outward therefrom is a rear ferrule 22 configured as shown with a tapered nose portion 24 and a rear flange 26 having an inclined end surface 28. The inclined end surface of the rear ferrule 22 provides a radial component as well as an axial component of the pull-up forces acting on the end surface as will be apparent to those skilled in the art. The tapered nose 24 enters a tapered camming surface in the rear surface of the front ferrule.
The ferrules 16, 22 are enclosed by a drive nut member 30 threaded to the body 10. During tightening and make-up of the fitting, the inner end face, flange, or shoulder 32 of the nut acts against the rear wall end surface 28 of the rear ferrule to drive the ferrules forwardly into the fully engaged position shown in FIG. 1A.
The small diameter portion or nose of the rear ferrule is dimensioned so that it plastically deforms during make-up of the fitting. This action is desirable since it results in tight gripping engagement of the outer wall of the tubing. The thickness of the nose portion cannot be reduced to an extent that the rear ferrule deforms too much and only the rear ferrule adequately grips the outer wall of the tubing. That is, the two ferrule assembly requires desired deformation of both the front and rear ferrules for the gripping and sealing capabilities that have made this two ferrule assembly a commercially successful product. On the other hand, the thickness of the nose of the rear ferrule cannot be enlarged to such an extent that it results in a structural arrangement that is too stiff and does not permit the desired rear ferrule deformation.
A more complete description and understanding of the conventional two ferrule phase controlled sequential gripping action resulting from the inclined rear surface and the interaction of the front and rear ferrules is set forth in U.S. Pat. No. 3,103,373 issued to Lennon, et al., the entire disclosure of which is fully incorporated herein by reference.
Accordingly, it will be recognized by those skilled in the art that a predetermined wall thickness of the nose of the rear ferrule is desired that achieves the desired gripping of the tube and cooperates with the front ferrule in such a manner that it achieves its desired goals of gripping and sealing the tube.
It is also recognized that operators of fluid systems test the system prior to a production run by pressurizing the system to an appropriate factor times the rated system pressure. In this manner, the operator can easily detect whether the fluid system is sealed, i.e. that there are no leaks. With this knowledge, the manufacturer can provide a fitting in which the nose of the rear ferrule will not have any additional plastic deformation at the elevated test pressure. Accordingly, the elevated test pressure is used to determine the desired wall thickness of the nose portion of the rear ferrule to achieve the desired amount of deformation of the nose and permit the front and rear ferrules to properly grip and seal with the outer wall of the tube.
It has also been found that galling of the drive nut sometimes occurs in the drive face area of engagement between the inner end face of the drive nut and the rear wall of the rear ferrule. After analysis, it is believed that the axial thrust or pull-up force between the front and rear ferrule is essentially parallel to the axis of the fitting. This axial thrust causes the rear comer region of the rear ferrule to selectively concentrate pull-up stress at the inside drive surface of the nut particularly in a localized area to produce the galling. This also noticeably increases the nut torque forces experienced during make-up even if galling is absent. Accordingly, it would be highly desirable to provide a design wherein the thrust forces do not produce the high localized loading with the resultant galling and high torque forces.
In accordance with one embodiment of the invention, a tube fitting includes a fitting body having a cylindrical bore for receiving a tube end and including a tapered mouth at one end of the bore; a drive member having a threaded engagement with the body and having a ferrule drive surface; a first ferrule having a tapered first end that extends into the tapered mouth of the fitting body and having a second end with a tapered recess that axially extends toward the first end; and a second ferrule having a cylindrical interior wall, a tapered first end that extends into the tapered recess of the first ferrule, and having a contoured face on a second end thereof that engages the drive member ferrule drive surface; the second ferrule interior wall having a circumferential recess located between the first and second ends of the second ferrule; the recess and the contoured face reducing stress concentrations on the drive member drive surface when the fitting is made up.
Another aspect of the invention includes forming the rear ferrule with a cylindrical interior wall that has a first diameter at the forward or nose end of the ferrule and a second diameter at the rear end of the ferrule wherein the second diameter is greater than the first diameter so that, for example, a single tool can be use to form the circumferential recess and the cylindrical wall. Still a further aspect of the invention includes forming a notch on the outside diameter of the ferrule body that joins the second ferrule first and second ends. In another aspect of the invention, a contoured drive surface is provided on the drive member rather than on the rear ferrule second end. The present invention may also be practiced with ferrules that are made of harder base metal than prior ferrules. The invention may also be practiced with ferrules that have been case hardened either entirely or selectively on the ferrule surface. In general, the present invention may be used in single ferrule fittings as well.
These and other aspects and advantages of the present invention will be apparent to those skilled in the art from the following description of the preferred embodiments in view of the accompanying drawings.